The present invention relates generally to capstan servo systems used in recording and/or reproducing apparatus, and more particularly to a capstan servo system used in an apparatus capable of selectively recording and/or reproducing either NTSC system video signals or CCIR system video signals even though the video signals of these two systems have different frame frequencies.
In an apparatus for recording/reproducing video signals, a capstan servo system is generally adapted for stabilizing tape travel. The capstan servo system known heretofore generally comprises a detector for detecting the rotation of the capstan, a phase comparator for comparing the detected rotation signal and a phased reference signal. A frequency discriminator frequency discriminates the rotation signal. The rotation of the capstan motor is controlled in response to the sum signal of the phase comparison error signal and the frequency discrimination signal.
The NTSC system video signal has a frame frequency of 30 Hz while the CCIR system video signal has a frame frequency of 25 Hz. Accordingly, the frequency of the reference signal supplied to the above mentioned phase comparator differs depending upon the frame frequency of the system of the video signal which is to be recorded or reproduced. On the other hand, the tape travel speed must be the same irrespective of the system of the video signals to be recorded or reproduced. Therefore, there is a necessity for replacing the rotation detector and the frequency discriminator to conform with the system of the video signals recorded or reproduced.
However, it is extremely difficult or almost impossible for the operator to carry out the above described replacement operation in actual practice.
As another approach to the problem, it is possible to provide an apparatus wherein two sets of detectors and frequency discriminators are used to conform with the respective video signal systems. In such an apparatus, the operator would set a switch to put the detector and frequency discriminator, appropriate for the video signal system, into operation. This apparatus having two detectors and two frequency discriminators would be expensive and would not be practical. Moreover, in this case, as in the case of producing separate models for the NTSC and CCIR systems, the manufacturer would be burdened with the necessity of fabricating and building into his products two kinds of detectors and two kinds of frequency discriminators, thus increasing the number of components he would have to procure and stock.